Steve Schatz (left) and Dr. Thomas Nuckton prepare for an afternoon swim off San Francisco's Aquatic Park last week. Both men are members of the Dolphin Club, which encourages swimming without wet suits.

Dolphin Club and Polar Bear Club member, Dr. Thomas Nuckton, on Thursday Jan. 10, 2013, shows the people that were used in the study, at the Dolphin Club in San Francisco, Calif. Dr. Thomas Nuckton is the author of a new study, Body Composition of Cold-Water Swimmers: The San Francisco Polar Bear Swim study.

Photo: Michael Macor, The Chronicle

Dolphin Club and Polar Bear Club member, Dr. Thomas Nuckton, on...

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Dr. Thomas Nuckton measures fellow bay swimmer Ralph Wenzel at San Francisco's Dolphin Club. Nuckton recently determined that club members were about the same size as average Americans.

Photo: Michael Macor, The Chronicle

Dr. Thomas Nuckton measures fellow bay swimmer Ralph Wenzel at San...

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Schatz (left) and Nuckton brave the bay waters. Nuckton, a physician, has been studying his fellow cold-water swimmers.

Photo: Michael Macor, The Chronicle

Schatz (left) and Nuckton brave the bay waters. Nuckton, a...

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Dolphin Club and Polar Bear Club members, Steve Schatz, (left) and Dr. Thomas Nuckton emerge from the Bay waters after an afternoon swim off of Aquatic Park, on Thursday Jan. 10, 2013, in San Francisco, Calif. Dr. Thomas Nuckton is the author of a new study, Body Composition of Cold-Water Swimmers: The San Francisco Polar Bear Swim study.

Photo: Michael Macor, The Chronicle

Dolphin Club and Polar Bear Club members, Steve Schatz, (left) and...

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Dolphin Club and Polar Bear Club members, Era Osibe, (left) and Dr. Thomas Nuckton demonstrate how the data was collected here recording neck measurements on Thursday Jan. 10, 2013, at the Dolphin Club in San Francisco, Calif. Dr. Thomas Nuckton is the author of a new study, Body Composition of Cold-Water Swimmers: The San Francisco Polar Bear Swim study.

Photo: Michael Macor, The Chronicle

Dolphin Club and Polar Bear Club members, Era Osibe, (left) and Dr....

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Dolphin Club and Polar Bear Club members, Steve Schatz, (left) and Dr. Thomas Nuckton emerge from the Bay waters after an afternoon swim off of Aquatic Park, on Thursday Jan. 10, 2013, in San Francisco, Calif. Dr. Thomas Nuckton is the author of a new study, Body Composition of Cold-Water Swimmers: The San Francisco Polar Bear Swim study.

Last week, the Bay Area was quivering through a cold snap, and the tourists along Fisherman's Wharf were, for once, dressed for the weather, bundled in jackets and scarves, mittens and caps.

Down at Aquatic Park, Dr. Thomas Nuckton was stripped to his swimsuit and neck deep in 50-degree bay water. And he wasn't alone.

Winter is the time of year that the truly devoted - or truly crazy - swimmers come out, those hardy folk who dive nearly naked into San Francisco Bay, into waters 25 or 30 degrees colder than the average swimming pool.

The health benefits of these swims are debatable, the risks less so. And yet every year, dozens, maybe a hundred, people swim all through the winter months, putting in miles and hours in the water.

"People will go down there to swim, and it's almost like a spiritual experience," Nuckton said.

"It's not quite the same for me. I hate getting in the water," he added with a laugh. "But I have my good days. And I'd much rather be there than in a pool, staring at a black line and just going back and forth."

Nuckton is a doctor at UCSF and California Pacific Medical Center, focused primarily on studying survival prognoses for patients in intensive care wards. But he's taken a side interest in swimming and, in particular, the effects of cold-water dips.

Aided by extra fat?

Nuckton's most recent study was prompted by a comment he overheard from a tourist standing on the Hyde Street Pier, watching the Aquatic Park swimmers. "He pointed at the swimmers and said, 'They're fatter than us. That's how they endure the cold,' " Nuckton recalled.

It's a commonly held belief, by swimmers and landlubbers alike, that an extra layer of fat insulates regular bay swimmers from the deep chill of the water. But Nuckton, a fairly trim swimmer himself, wasn't so sure the tourist was right.

Last month, after a yearlong study of people in his swimming club, Nuckton was able to show that these swimmers are, in fact, just about the same size as the average American. Crazier, maybe. But not fatter.

What that means is that cold-water swimmers probably aren't any better protected from the elements than anyone else. Plus, at least in the Bay Area, most of them aren't elite-level athletes either. But something about them makes them better able to tolerate and overcome the cold.

"All of these factors probably come into play," Nuckton said. "How good is your engine in terms of generating heat? How cold is the water and how long are you going to spend in it?"

Over the past decade, Nuckton has made a hobby of studying the members of his swim group, the Dolphin Club. Along with the South End Rowing Club, the Dolphin Club encourages no-wet-suit swimming in the bay. The clubs' members host several group events year-round, including a New Year's Day swim from Alcatraz to Aquatic Park, and they operate hot showers and saunas for post-swim recovery.

More fit, but why?

There is no doubt, doctors say, that regular cold-water swimmers are healthier overall than most Americans. But whether it's the cold or the simple act of regular exercise that helps them is unclear.

There are theories, but not much evidence, about how cold water can improve health. Some studies have shown that people who swim through the winter have higher immune cell counts than nonswimmers. And some scientists, and swimmers, have speculated that cold-water swimming increases production of what's known as brown fat, thought to be a healthy fat that burns energy and creates heat.

Many cold-water swimmers believe they're burning more calories in the bay as their bodies struggle to stay warm than they would in a heated pool, but scientists aren't convinced that's the case. Cold water, in fact, slows the metabolism, which may mean that the body is actually burning fewer calories. Even if there is a larger calorie burn, it's probably negligible, doctors say.

"I'm not convinced that, physiologically speaking, cold-water swimming helps. I think these people are already healthy to begin with and they want to challenge their bodies," said Dr. Malini Singh, medical director of emergency medicine at San Francisco General Hospital.

"I know people do it. Anecdotally speaking, they say they feel refreshed, they feel less stress and less fatigue," she added. "But I've been in that water, in a wet suit, and I just felt terribly cold to my bones."

Risks overstated

It's not just the benefits of cold-water swimming that are tough to document. The risks are poorly understood and probably overstated too, some doctors said.

Two studies of athletes competing in long-distance, open-water swims found that most of them were suffering mild hypothermia by the end of the race, even when the water temperatures were in the 60s or higher.

Nuckton studied 11 participants in a 2000 Alcatraz swim and found that five were suffering mild hypothermia by the end of the event. And perhaps more concerning, their temperatures continued to drop after they got out of the water.

But mild hypothermia on its own is not necessarily a dangerous thing. In Nuckton's study, most swimmers' temperatures dropped about 2 degrees below normal, which probably wouldn't produce any noticeable symptoms or discomfort. In order for hypothermia to cause major damage or become deadly, the body temperature needs to drop to the low 90s.

The concern with hypothermia is twofold. First, that someone may experience early symptoms of it and become sluggish or disoriented, which could be deadly when a swimmer is in deep water and has trouble making it safely back to land. And second, some older or less fit swimmers may not be as well adapted to handling the cold as their younger or more experienced peers.

"The important thing is, I wouldn't be doing this for the first time, just jumping right in, if you're elderly or have significant medical issues," said Dr. Joel Levis, assistant chief for the emergency department at Kaiser Santa Clara. "Those people are better off just swimming in a pool."

Peril of sudden shock

Aside from hypothermia, there is a small risk associated with the sudden shock of jumping into cold water. Anyone who has dived into frigid water knows the feeling - the heart rate soars, breathing becomes erratic, and it usually takes a minute or two to calm down. That's rarely a problem, but there have been a handful of cases of people suffering heart attacks and subsequently drowning from that initial shock.

And then there's the native aquatic life. For the most part, swimmers exist companionably with the critters, but several years ago a rogue sea lion bit dozens of people in Aquatic Park, requiring a few of them to get stitches and antibiotics.

But on the whole, a swimmer is probably more likely to be injured by someone doing a sloppy butterfly stroke at the pool than by, say, a great white shark in the open water.

"The chances of getting eaten by a shark are pretty low. You just don't think about it," said Robin Rose, an Aptos resident who swims regularly in Santa Cruz, usually doing a mile around the wharf several times a week.

Rose started open-water swimming in 2009, and she swam regularly even through treatment for breast cancer in 2010. She wears a wet suit for now - but she's seriously considering joining the "skin" swimmers sometime in the near future.

"I went to Aquatic Park, just to try it, and it actually felt really good," Rose said. "I was only in for about five minutes, and it was so cold, but it was so invigorating. There's nothing else like it."